The recognition event between T cells and I region encoded determinants on syngeneic non-T cells has been termed the autologous mixed lymphocyte reaction (AMRL). A T cell factor(s) recoverable in the supernatant of AMLR cultures can mediate the generation of hapten-directed cytotoxic T cells. The AMLR helper factor has no activity on an IL-2 dependent cell line, nor does it function in either PHA or Con A constimulator assays. Unlike IL-2, the AMLR helper factor also fails to activate thymus cytotoxic precursors in the presence of antigen. Thymic adherent cells are necessary intermediates for the AMLR helper factor to manifest its activity. These preliminary data suggest that the AMLR helper factor may be important in the initiation of the cytotoxic response. The proposed studies will more closely examine the role of the AMLR helper factor in the cytotoxic T cell activation sequence. The H-2 restriction of the factor in the generation of cytotoxic activity, as well as the MHC restriction and antigen specificity of the cytotoxic cells will be studied. The response to a variety of haptens plus self, H-Y antigens, and allodeterminants in primary and secondary responses will support a general role for the factor in the cellular immune response. The effect of the AMLR factor on IL-1 and IL-2 production will affirm its position in the cytotoxic induction pathway. The cell interactions required for the production of the AMLR helper factor will also be examined. Splenic T and non-T subpopulations will be analyzed for their role in the production of the AMLR helper factor. The T cell subset will be characterized and its relationship to the AMLR proliferating cell will be clarified. These studies will have direct bearing on the nature of the T cell defect in autoimmune strains of mice. Preliminary studies suggest that autoimmune NZB mice have a T cell defect in the cell which produces IL-2, but still produce the AMLR factor. Finally, biochemical and biophysical studies will provide data on the molecular characteristics of the AMLR helper factor. These results, in concert with the biological assays which detect many of the cytokines, will identify the AMLR factor as a known mediator or as a novel immunoregulatory agent.